There has been already proposed a compensating apparatus for compensating reactive current components and higher harmonic components of a load current of a power system by connecting an inverter to the power system to supply a compensation current thereto. FIG. 1 shows a current command calculating apparatus in such a compensating apparatus, used with an inverter for
In FIG. 1, in accordance with a load current i.sub.L of a power system detected by a current detector (not shown) and a system voltage V.sub.s, i.e., in accordance with a so-called instantaneous p-q theory, a PQ calculation circuit 3 as a power vector calculation circuit calculates an instantaneous real power P and an instantaneous imaginary power Q. The instantaneous p-q theory is described in detail in "Generalized Theory of Instantaneous Reactive Power and its Application" by Hirofumi AKAGI, et al. Papers B of the Institute of Electrical Engineers of Japan, Vol. 103, No. 7, pp. 483 to 490 (1983).
A power calculation circuit 4 derives components to be compensated, from an output of the PQ calculation circuit 3. The power calculation circuit 4 derives higher harmonic active power components P.sub.h, fundamental wave reactive power components Q.sub.o, and higher harmonic reactive power components Q.sub.h from the instantaneous real power P and instantaneous imaginary power Q. In accordance with the derived components P.sub.h, Q.sub.o, and Q.sub.h, and the system voltage V.sub.s, an inverse PQ calculation circuit 6 as an inverse calculation circuit calculates a current command i.sub.c.sup.* which is expressed by ##EQU1##
An absolute value limiter 7 clamps the current command i.sub.c.sup.* to such a magnitude that makes an output of a compensation current supplying inverter not exceed its rated value. The clamped current command i.sub.c.sup.*, is supplied to the inverter.
Since the absolute value limiter 7 shown in FIG. 7 clamps the i.sub.c.sup.* to such a current command i.sub.c.sup.*, that makes an inverter output current not exceed its rated value, there occurs wave saturation, i.e., distortion caused by wave clamping, posing the problem that the current command i.sub.c.sup.*, includes higher harmonics.
FIG. 2 is a block diagram showing another conventional current command calculating apparatus. Instead of the absolute value limiter 7 shown in FIG. 1, a peak detector 16, inverse function calculation circuit 17c, and multiplier 19 are used to configure a so-called auto-gain-control circuit. The current command i.sub.c.sup.* outputted from the inverse PQ calculation circuit 6 is supplied to the multiplier 19 as one input thereof, and is also supplied to the peak detector 16. The peak detector 16 detects and holds the peak value of the current command i.sub.c.sup.*. The inverse function calculation circuit 17c calculates a gain k if the peak value held by the peak detector 16 exceeds a rated value of the inverts. The gain k is determined through an inverse function calculation so that the current command does not exceed the inverter rated value. The calculated gain k is supplied to the multiplier 19 as the other input thereof. The multiplier 19 multiplies the current command i.sub.c.sup.* by the gain k to output the current command i.sub.c.sup.*, to the inverter.
In this auto-gain-control circuit shown in FIG. 2, higher harmonic components and fundamental wave reactive power components are uniformly limited. Therefore, even if either the higher harmonic components or the fundamental wave reactive power components exceed a limit value, both the components are limited, thereby posing the problem that the performance of the inverter cannot be effectively used.